Operation Hubble

Posted by cafuego on Sunday 5 May 2013.

I got a telescope a few years back and though it works well for looking through with human eyes, it's been close to impossible to use with with a digital SRL camera mounted at the eyepiece. The problem is that the camera body can't move close enough to the tube to obtain focus on objects futher away than about 20 metres. Of course, that's not very useful for a telescope (unless you'e into bird-watching).

The camera can be made to focus with the addition of a barlow lens, but the only one of those I have magnifies by a factor of two and adds some blurring, so that's not really an ideal solution either. What I really want is to put the camera at prime focus using only the primary and secondary mirror.

On one of my bi-annual google searches for a solution I stumbled across the suggestion of a Hubble style operation to mount the telescopes primary mirror a bit closer to the secondary mirror, so making the focal plane move a bit further away from the tube. However, the original post is rather low on detail.

From the images added to the original post, it looked like the poster had used book binding screws known as "chicago screws" or "sex bolts" (not be confused with Andrew) to replace the thumb screws on the end of the telescope, to give the mirror assembly more inward travel and longer springs to prevent vibration.

Components

I found chicago screws at a craft store, but they turned out to be a bit short and made it nigh impossible to collimate the mirror by giving close to no purchase (compared to the thumb screws).

On my quest to find a matching longer screw head for the chicago screws, I ended up at a hardware store where one of the clerks actually found some used 5mm × 45mm machine screws that appeared perfect for my needs, but unfortunately he couldn't find any springs to match.

A bit more googling on the tram home though, led me to the RS Components website, which lists a plethora of varied size and strength springs. Including one that appears to fit :-) I ordered a set and a few days later I had everything I needed for my Hubble style telescope surgery.

Disassembly

To remove the primary mirror assembly, unscrew the small black screws from the bottom of the telescope (you can stand it on its front for this) and carefully lift the entire assembly out of the tube. Unfortunately, the screws that keep the assembly attached to the backing plate are half obscrured by the rpimary mirror, so that will need to be removed too. You need a small screw driver to carefully undo the six screws that keep the mirror in place. Carefully lift the mirror off, put it in a safe place and cover it to keep dust off.

Remove the thumb screws and the backing plate, then turn the assembly over. You can now remove the screws that attache the mirror assembly to the backing plate and replace them with your longer ones.

Provided you got the correct springs (mine are 11mm diameter, 56mm long, 1mm piano wire), they should fit perfectly and push the mirror away from the backing plate with a fair bit of force. Add the backing plate and put the thumb screws back on. You may need to compress the springs quite firmly to accomplish this.

When that's done, all that remains is to reinstall the mirror and gently reinsert the whole assembly back into the tube.

Insert error

When I performed this last step I found that it was close to impossible to insert the mirror back into the tube. On closer inspection, some scratches on the mirror assembly implied it was catching on the small screws that keep the end cap in place. These would appear to be just the slightest bit too long. D'oh!

I wasn't about to go off again and find some more screws, so instead I simply reversed these. The screw head is now on the inside of the tube and the hex nut is on the outside, allowing the mirror assembly free travel. If you do this, just be sure to not have any clothing catch on that screw when you're out in the field.

I've not yet had the time to properly try this new setup to see if it makes any difference to focusing, fingers crossed!

Comments

The springs are just over $3 a piece and come in a packet of 10 minimum, so I have 7 left at the moment. If you're in Australia, have this scope, are thinking of giving this operation a try and don't want to spend $35 on more springs than you need, ping me on twitter.